This invention relates to tensioners for threaded members.
Tensioners are commonly used for, amongst other tasks, stretching items such as bolts or studs, so that a threaded nut can then be run down the stretched bolt, thus capturing the tension in the stretched bolt. Typically, they are used in any field that requires large loads to be secured by the use of threaded fasteners; one common use is in wind turbines, when such tensioners can be used to secure many fastenings, such as the turbine blades to their bearing, or the tower of such a turbine to its foundations.
It is desirable to know the elongation of the threaded member, particularly in situations where the traceability of the attachment of the nut on the threaded member is important, such as in the example of wind turbines discussed above. Whilst it is, to some extent, possible to calculate the elongation of the threaded member by using the pressure of the hydraulic fluid introduced into the pressure space in combination with the material dimensions and properties of the bolt, this is however not a particularly reliable measure.
It is also possible to measure the elongation of the threaded member ultrasonically; a fixed ultrasound transducer emitting ultrasound waves onto the end of the threaded member can measure the distance to the end of the threaded member by measuring the time of flight of the reflected ultrasound waves off the end of the threaded member. However, such equipment is cumbersome to set up and is difficult to calibrate.
Hydraulic tensioners are well known in the art; examples of the same can be seen in the European Patent Application published as EP 2 522 465 and in the Japanese Patent Application published as JP3-204406. Such tensioners can be used to stretch a threaded member such as a stud, bolt or the like, and generally comprise inner and outer coaxial generally cylindrical annular bodies which are placed around the threaded member. The inner body threadedly engages the threaded member. A space between the inner and outer bodies defines a pressure space, into which fluid, typically hydraulic fluid, can be introduced to drive the bodies apart along their common axis.
It has been proposed, in the two patent application publications to which reference is made above, to use a special threaded member which has a central bore. A rod is placed into the central bore, and a measuring apparatus clamped to the end of the threaded member. The movement of the measuring apparatus relative to the rod then gives a measurement of the elongation of the threaded member. However, this again requires access to the end of the threaded member and requires significant clearance at the end of the threaded member.